There are two general types of ink jet printing, drop-on-demand (DOD) and continuous (CIJ). Drop-on-demand printing, as its name suggests, produces droplets of ink as and when required in order to print on a substrate. Continuous ink jet printing, to which the present invention relates requires a continuous stream of ink which is broken up into droplets which are then selectively charged; either charged or non-charged droplets are allowed to pass to a substrate for printing, charged droplets being deflected in an electric field either on to the substrate or into a gutter (according to design) where the non-printed droplets are collected for re-use. In the first case, the droplets are deflected by an electric field onto the substrate with the uncharged drops going straight on to be collected in a gutter for re-use. The amount of charge also determines the relative printed position of the drops. In the second case, the droplets are deflected into an offset gutter, with the printing drops being the uncharged ones going straight onto the substrate. The obvious advantage of printing with the uncharged drops is that, in a multi-jet printer where several drop generators are aligned perpendicular to a moving substrate, the alignment of the drops printed on the substrate is not dependent on the ability to accurately and uniformly charge the drops. As long as the charge on the droplets is sufficient for the drops to be deflected into the gutter aperture, small variations in the charge applied will not affect the quality of the resulting print. This second type of printer is generally known as a binary jet printer as the droplets are either charged or uncharged (and do not intentionally carry varying amounts of charge that determine print position).
In typical continuous ink jet printers, the print head has a droplet generator which creates a stream of droplets of ink by applying a pressure modulation waveform to the ink in a cavity in the print head and the continuous ink stream leaving the print head breaks up into individual droplets accordingly. This modulation waveform is usually a sinusoidal electrical signal of fixed wavelength. The stream of ink leaving the print head breaks up into individual drops at a distance tor time) from the print head commonly known as the break-up point, that is dependent on a n umber of parameters such as ink viscosity, velocity and temperature. Provided these and other factors are kept relatively constant, then a given modulation waveform will produce a consistent break-up length. In order to induce a charge on the droplet, the charging waveform must be applied to the stream at the moment before the drop separates from the stream, and held until the drop is free (ie. must straddle the break-up point). It therefore necessary to know the phase relationship between the modulating waveform and the actual drop separating from the stream (ie. during which part of the sinusoidal modulation waveform does break-up occur).
One method of determining this phase relationship involves a charge detector (and associated electronics), position somewhere after the charging electrode, which can detect which drops have been successfully charged. A half width charging pulse, progressively advanced by known intervals relative to the modulation waveform, is used to attempt to charge the droplets and the detector output analysed to determine correct charging. Because of the half width pulse, theoretically half the tests should pass and half should fail. The full width pulses used for printing would then be positioned to straddle the detected break-up point. The number of intervals that the waveform is divided into, and therefore the number of possible different phases, can vary from system to system, but usually the timing is derived from a common digital close signal, and therefore is usually a binary power (ie. could be 2, 4, 8, 16, 32 etc.). Typically, 2 and 4 intervals would not give sufficient resolution, and 32 intervals upwards would make the tests too time consuming. Using 16 intervals (ie. 16 different phases) is considered to give more than adequate accuracy without involving a detrimental number of tests.
In a multi-jet print, due to manufacturing tolerances of the nozzles and the characteristics of the (usually common) ink cavity, the break-up point for each of the streams, and therefore the phase setting for printing will be different.
Modern multi-jet printers, in order to be able to print high-quality graphics and true-type scalable fonts, utilise a large number of ink streams, placed very closely together (typically 128 jets at a spacing of 200 microns).
Although it has proved possible to manufacture charge electrodes at the required spacing, to individually charge the streams, it would not be practical to duplicate existing charge electrode driver circuitry 128 times, and so current trends lean towards the use of an integrated driver solution in which a large number of the drive circuits are implemented in one Integrated Circuit device, in order to save space, reduce power etc. With such a device, for practical reasons, it is not possible to enable, or set the level of charging voltage on an individual jet basis, and so all the high voltage drivers within the device have a common enable and common power supply.
Additionally, at present it is not possible to have a separate phase detector for each stream. The probability is that the individual detectors would never be able to isolate the charge from their own stream from the effects of any adjacent streams.
A final handicap to existing phasing methods being applied to this type of printer, is the fact that the "normal" condition for the droplet streams, ie. not printing, is for all the droplets to be charged. Therefore, to test individual jets would require the detection of the non-charged state, resulting in ink being sent to the substrate. Also, the phase detector circuitry would more than likely not be able to distinguish the change in charge passing the detector when a single jet was turned off, against a background of 127 jets still on.
Therefore conventional phase detection methods are not suitable for modern high-resolution binary CIJ printers.
In our British patent application no. 9626706.7 and our co-pending International patent application reference MJB05643WO we describe a method of phasing the jets at start-up which comprises generating a modulation waveform to operate the pressure modulator to cause droplets to be generated in each stream; and, independently for each group of charge electrodes:
operating the respective charge controller to supply a charge signal waveform to each charge electrode in turn; PA1 adjusting the phase of the charge signal waveform relative to the modulation waveform between 0 and 360 degrees in a number of steps; PA1 determining the optimum phase relationship to achieve proper charging for each droplet stream in turn; PA1 and thereafter adjusting the phase of the charge signal waveform relative to the modulation waveform to achieve charging of droplets in all the streams in the group simultaneously. PA1 generating a modulation waveform to operate the pressure modulator to cause droplets to be generated in each stream; PA1 operating the charge controllers to supply a charge signal waveform to the charge electrodes and charging droplets in the streams; PA1 setting the phase relationship of the charge signal waveform relative to the modulation waveform; and, PA1 to adjust the phase relationship of the charge signal waveform relative to the modulation waveform, during the printing process, when droplets do not require to be printed, independently for the charge controller of each group of charge electrodes, carrying out the steps of: PA1 (A) operating the charge controller to apply a DC PA1 voltage simultaneously to all the charge electrodes in the group to charge all the droplets to prevent printing; PA1 (B) applying a pulse signal waveform to the charge electrode controller, to reduce the amplitude of the DC voltage periodically and temporarily to a level below that of the DC voltage but still sufficient to cause droplets to be deflected to avoid printing; PA1 (C) sensing by means of a detector the aggregate level of charge applied to the droplets and generating signals representative thereof; PA1 (D) from the signals generated in step (C), determining the phase relationship of the pulse signal waveform relative to the modulation waveform; and, PA1 (E) if the pulse signal waveform is delayed relative to the modulation waveform, advancing the pulse signal waveform relative to the modulation waveform or, if the pulse signal waveform is advanced relative to the modulation waveform, delaying the pulse signal waveform relative to the modulation waveform, to reset the phase relationship of the pulse signal waveform relative to the modulation waveform.
Thus, for each group of nozzles/charge electrodes, the phase of the charge signal waveform is adjusted independently of that of the other groups so that proper charging of droplets in all the streams can be achieved.
However, the phase relationship also has to be maintained during printing over long periods and parameters such as temperature and ink viscosity change during printing. This has previously required the print head to be stopped frequently for readjustment as, hitherto, it has not been possible to carry out phasing without stopping and re-starting the printer. Now, because uncharged droplets are used for printing, the method used at start-up cannot be used during printing (more accurately in pauses between actual print cycles) because, otherwise, unwanted droplets would be sent to the substrate and printed since it is not possible to move the gutter into and out of the `catch-all` position in the short time between print cycles. Furthermore, the use of the half-width pulse waveform of the method exemplified in our British patent application no. 9626706.7, is not possible either since all non-printed droplets must be charged in order to be sent (deflected) to the gutter in its operative position and that waveform has segments in which there is no charge applied to droplets.
The present invention is directed towards overcoming these problems.